The mysteriously aberrant motion of the two Pioneer spacecraft in the distant solar system may not be so mysterious after all. Last month, an apparently inexplicable force prompted NASA scientists to speculate that Einstein's theory of general relativity might need some updating, but a new analysis suggests a more mundane cause: tiny gas leaks from the spacecraft control system.

Four weeks ago, a team led by John Anderson of NASA's Jet Propulsion Laboratory in Pasadena, California, announced that as the spacecraft travel away from the sun, they have been slowing more than gravity alone can explain, as if a mysterious force were pulling on them (ScienceNOW, 9 September[2]). The team tried to rule out all possible causes, leaving the prospect of a flaw in the laws of physics. But according to team member Philip Laing of the Aerospace Corp. in El Segundo, California, they have now noticed that minor variations in the anomalous speed change seem to have occurred as valves opened and closed during routine adjustments of the spacecraft's spin and orientation--suggesting that gas leaks are braking the craft. The team now suspects that other, more constant gas leaks may account for the bulk of the anomaly.

The same process is almost certainly affecting the European Ulysses spacecraft, which also appears to be pulled toward the sun. "Ulysses is leaking all over the place, probably because of sticking valves," says Laing. A few grams of leaking gas per year would be enough to slow the craft.

Jonathan Katz of Washington University in St. Louis, Missouri, has another, equally mundane explanation for the effect: He thinks it may be due to waste heat from the radioactive power source aboard the spacecraft. Heat radiation from the generator would be reflected by the backside of the communications antenna on each Pioneer, giving a tiny push toward the sun, says Katz. But Laing doesn't think this is happening. He notes that the radioactive decay of plutonium--and the heat waste--should decrease over time, while the mystery acceleration remained constant between 1980 and 1988.